Aerogels are porous materials obtained after gelling and then drying of the gel, in which the liquid acting as solvent has been replaced with a gas or gas mixture. At very low density (or at high pore volume) these materials are very promising for uses as thermal insulators. This is because their nanoporosity makes it possible to limit the effects of convection of the air contained in the pores.
The preparation of very low density aerogels is, however, complicated because of their limited mechanical properties, which to date do not allow conventional oven-drying, in particular because of the vaporization of the solvent during this oven-drying, which gives rise to internal stresses in the material, destroying its nanostructure and creating macrofissures therein. This is why drying with supercritical CO2 is conventionally used for the production of these low-density aerogels. This method gives good results regarding the stability of the nanostructure, but it has the drawback of penalizing the manufacturing cost of the aerogel.
Silica aerogels, which are the ones that have been most widely studied for applications as thermal superinsulators (these aerogels may have thermal conductivities of about 0.015 to 0.020 W·m−1·K−1), do not escape these findings. Thus, with conventional oven-drying, these silica gels undergo substantial densification and a loss of their nanostructure. Furthermore, the fissuring of these gels creates fines, which poses toxicity problems due to the release by the powder of silica nanoparticles. Research efforts have thus been concentrated on the spring-back effect of silica aerogels after modification of the chemical nature of their surface, and on replacement of the silanol groups with unreactive groups making it possible to render the densification reversible after evaporative drying.
This principle has allowed the industrial production of low-density silica powder in the form of a thermal superinsulating nanostructured aerogel, but has not allowed the synthesis of stable monolithic material, in contrast with organic aerogels of high specific surface area, which are, themselves also, promising for uses as thermal superinsulators.
In a known manner, these organic aerogels are typically prepared from a resorcinol-formaldehyde (RF) resin, which has the advantage of being inexpensive and of being able to give a gel used in water and of being able to have various porosity values and density values depending on the preparation conditions (according to the ratios between reagents R and F and the catalyst, for example). Furthermore, these organic aerogels can be pyrolyzed in the form of carbon with a high specific surface area having the advantage of absorbing infrared radiation, and thus of having a low thermal conductivity at high temperature. On the other hand, these chemical gels obtained by polycondensation of precursors are irreversible and therefore cannot be reused. Furthermore, at high conversion, these gels become hydrophobic and precipitate out, which induces mechanical stresses in these materials and increases their fragility.
As for silica aerogels, it is thus necessary, in order to obtain very low density organic monolithic aerogels, to use a drying technique that is mild enough to avoid fracturing or contraction of the nanostructure and a loss of specific surface area for these aerogels. This drying is conventionally carried out via solvent exchange with an alcohol, and then via drying by means of supercritical CO2.
Mention may be made, for example, of document U.S. Pat. No. 4,997,804 for the description of a process for manufacturing such an organic monolithic aerogel based on resorcinol-formaldehyde resin, which uses this drying by solvent exchange and then by supercritical fluid.
As previously indicated, a major drawback of this drying technique is that it is complex to perform and very expensive.